Hey there - first post ever, anywhere. I do alot of engine & automotive research online - this site is great for providing good information by people with experience.
So here's one I've been wondering about, hopefully it's not out of place on this forum, since it pertains to engine efficiency and great driveability not necessarily maximum horsepower and torque (I know they aren't mutually exclusive either).
Here's the scenario : A Gen V 454 running running on propane is going into a heavy, tall geared Suburban. No towing and no racing. The main objectives are part throttle torque, responsiveness, and fuel efficiency. Another limiting factor is that a high piston dome is being avoided because the dome itself contributes to less efficient flame travel. Question: If you had to choose between the high port velocity of the stock heads, but somewhat lower that optimal compression, or larger ports with less velocity but optimal compression, which would you choose and why? I might be splitting hairs avoiding the piston dome and sacrificing compression, but still wonder about the question. Thoughts?
Curt

Hey there - first post ever, anywhere. I do alot of engine & automotive research online - this site is great for providing good information by people with experience.
So here's one I've been wondering about, hopefully it's not out of place on this forum, since it pertains to engine efficiency and great driveability not necessarily maximum horsepower and torque (I know they aren't mutually exclusive either).
Here's the scenario : A Gen V 454 running running on propane is going into a heavy, tall geared Suburban. No towing and no racing. The main objectives are part throttle torque, responsiveness, and fuel efficiency. Another limiting factor is that a high piston dome is being avoided because the dome itself contributes to less efficient flame travel. Question: If you had to choose between the high port velocity of the stock heads, but somewhat lower that optimal compression, or larger ports with less velocity but optimal compression, which would you choose and why? I might be splitting hairs avoiding the piston dome and sacrificing compression, but still wonder about the question. Thoughts?
Curt

I'd chose compression.
I have a lot of respect for the power of mixing/turbulance, wet flow, and so forth, being more important than most say, but in this case I think aftermarket heads or piston domes will work fine as long as they are reasonable, not overkill.
Dry fuel is not so picky about wet flow.

3X
'Propane', depending on the actual composition, is 100+ octane, but as a gas, it displaces a significant percentage of charge air, thus losing power and torque, so you want all the compression you can get. I expect the stock CR is not more than 8.5:1 so a medium dome and perhaps milling the heads might get you up to 10.5 or 11:1, yielding a significant boost in power and torque.

Thanks for the input!
My understanding is that these small heads with high port velocity will contribute greatly to low speed torque and response. I like the surge of torque when you add a little throttle at low rpm, for instance just after an upshift. High port velocity in the small heads contributes to that I believe. Again, I might be splitting hairs, and a 30 or 40 cc larger port might feel equally great to me on an engine as big as a 454. That's why I'm throwing the question out there, to check with others about what you've experienced. The fact that these peanut ports are the heads I already own is a big plus too. I can achieve the compression I want with a dome piston, and might still decide to go that route and see what happens - I'm sure it wouldn't be terrible.
My intent is to use it as my daily driver, around town parts chaser, people hauler and highway cruiser. The small block in there now does just fine, but I've decided I'm over it. I used to run big blocks back in the day, and now I want that easy big block power in my life again. And I just want to open my hood & see a big block. Just because.
Thanks again.
Curt

I actually ran across a set of peanut port bbc heads that came out of a stationary gen. They had hardened seats on intake and exhaust for natural gas I believe. Seats got machined to 2.19"/1.88" something like that would get the job done with the slight dome piston and last. Thanks, Charlie

You're on to me!
Yes I drive a 4WD 88 Suburban 'brick'. It's what I have and what I can afford. And I'm enjoying tinkering with it to make it better.
If I could, I'd have already ordered some AFR 265 heads, a 496 rotating assembly, etc,etc. That would be fun, and a relative 'no brainer', but it's just not an option right now. Besides, the challenge of improving what I have without spending much money is kinda fun for me.

Thanks again for the responses everyone, the input is appreciated.
mag2555, you mentioned weight in reference to the 4wd system. In my "faster for free" quest with this truck, I did put it on a serious diet, removing unnecessary weight. I've actually scraped off about 40 lbs of dirt and old grease that had accumulated over the years. Total weight reduction at this point is close to 400 lbs. Even in a big vehicle like this, it has made a very noticeable difference in how it feels to drive.

I'd chose compression.
I have a lot of respect for the power of mixing/turbulance, wet flow, and so forth, being more important than most say, but in this case I think aftermarket heads or piston domes will work fine as long as they are reasonable, not overkill.
Dry fuel is not so picky about wet flow.

+1
Compression has a direct, measurable and theoretical impact on the PV curve, which is the work extracted from each power cycle. Lower compression ratios necessitate late EVO events to extract the energy of the fuel burn by allowing the piston to travel farther. Look at it this way. Work is force times distance. If you lower the force, you have to increase the distance to get the same work. If you increase the force, you can decrease the distance. (Or keep the distance the same and get big improvements).

Also, engines are a total package. You can't just go black and white and say dome or no dome. For instance, take it to the extreme. Close down the the chambers like crazy to bump compression and avoid a dome. What have you done to chamber efficiency? You probably killed it. In a stock valve angle BBC, the best combo likely lies in a small, mostly unobtrusive dome and more open chamber.

Great illustration of a really big dome/bad chamber. This is a 17:1 Venolia from a set I had made for a Pontiac 4 cyl. Iron Duke running on CNG. The intent was to explore the knock limit. (Pure methane has a octane number of >120.)

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